"convert_bool" in gexf.py does not handle all bools

in gexf, line 820-823:
```
                atype=gexf_keys[key]['type']
                value=a.get('value')
                if atype=='boolean':
                    value=self.convert_bool[value]
```
calls line 211:
```
    convert_bool={'true':True,'false':False}
```
However, values such as "True" will also evaluate as boolean (on line 822) but are not in the convert_bool dictionary resulting in the error:
```
KeyError: 'True'
```